Matrix metalloproteinases (MMPs) are a group of 26 endoproteases that cleave components of the extracellular matrix. MMPs exist in their inactive zymogen (proMMP) forms, requiring activation by disruption of the complex between the cysteine residue in the prodomain and the zinc atom in the catalytic domain. MMP activity is regulated predominantly by endogenous inhibitors called tissue inhibitors of metalloproteinases (TIMPs). MMPs play roles in a wide variety of processes, ranging from cell death, differentiation, proliferation, cell signaling and migration, angiogenesis, wound healing, and tissue remodeling. When the activities of MMPs are imbalanced or uncontrolled, MMPs may play important roles in pathological processes, such as tumor metastasis and inflammation. MMPs also play roles in the development and repair of the central nervous system (CNS), as well as in the pathology of many neurological diseases.
Numerous studies implicating MMPs in cancer pathology resulted in evaluation of broad-spectrum MMP inhibitors in clinical trials in patients with advanced cancer. The compounds tested contained hydroxamate groups that chelate to zinc, and as such, inhibit MMPs and often other zinc-dependent enzymes broadly. Clinical trials with these broad-spectrum MMP inhibitors failed to extend survival. Moreover, toxic side effects, such as musculoskeletal pain and inflammation, were observed. The toxicities were attributed to the poor selectivity of the inhibitors. In addition, broad-spectrum MMP inhibitors advanced to clinical trials without adequate target validation. It is now recognized that some MMPs are essential for tumor progression and metastasis, but others play host-protective functions. Thus, the strategy of broad inhibition of MMPs is problematic. Numerous studies indicate that neurological conditions would benefit from MMP inhibitors, however selective MMP inhibitors are necessary. This is critical, as disparate MMPs mediate different roles. Some exert the desirable repair functions in disease, yet others promote the deleterious pathological consequences of neurological diseases.
A well-studied subgroup of MMPs is the gelatinases: MMP-2 (gelatinase A) and MMP-9 (gelatinase B). ProMMP-2 is present constitutively and is activated by MMP-14, while MMP-9 is inducible and is activated by MMP-3, plasmin, or under oxidative stress conditions. Gelatinases, in particular MMP-9, play roles in many pathological CNS conditions, with disruption of the blood-brain barrier (BBB) occurring in many neurological diseases. Following cerebral ischemia, activation of MMP-2 leads to disruption of the BBB, followed by a second wave of damage to the BBB after reperfusion, which is mediated by MMP-9.
Treatment with the selective gelatinase inhibitor SB-3CT (compound 1) rescues laminin from proteolysis and neurons from apoptosis following transient focal cerebral ischemia and protected the neurovasculature from embolic focal cerebral ischemia. Disruption of the BBB is observed after traumatic brain injury, which has been attributed to MMP-9 and aquaporin-4 (AQP4). Selective inhibition of MMP-9 with compound 1 attenuated secondary damage resulting from severe traumatic brain injury in mice. In mice expressing mutant superoxide dismutase (SOD1) that causes amyotrophic lateral sclerosis, reduction of MMP-9 by gene ablation, viral gene therapy, or chemical inhibition with the tetrapeptidyl hydroxamic acid FN-439, delayed muscle denervation, indicating that MMP-9 plays a major role in motor neuron degeneration. After spinal cord injury, elevated MMP-9 in the lumbar cord contributes to failure of motor relearning in mice and deletion of MMP-9 reduces inflammation in the lumbar cord and results in improved recovery.
MMP-9 also plays a role in epilepsy. In pentylenetetrazole-induced epilepsy, sensitivity to epileptogenesis decreases in mice lacking the MMP-9 gene and increases in rats overexpressing MMP-9, and MMP-9 deficiency diminishes seizures. MMP-9 significantly contributes to cell death after pilocarpine-induced seizures in the developing brain. Treatment with the broad-spectrum MMP inhibitor GM6001 mitigates cell death in pilocarpine-induced seizures in immature rats, and in the pathophysiology of brain injury following seizures. MMP-1, MMP-3 and MMP-9 have been implicated in BBB disruption in West Nile virus encephalitis. Treatment with the broad-spectrum MMP inhibitor GM6001 reversed West Nile virus-induced BBB disruption. These studies implicate gelatinases in various pathophysiological processes in the CNS. Thus, selective potent inhibitors of gelatinases that cross the BBB are highly sought.
Development of therapeutics that target CNS diseases requires that the drugs be delivered to the target site, the brain. However, the BBB is a major challenge to the development of CNS-active small-molecule therapeutics, constituting a physical barrier that prevents the transport of substances from the blood into the CNS. Small molecules are transported across the BBB by lipid-mediated transport, if they have a molecular weight of less than 400 Da and/or high lipid solubility. In practice, a very small number of drugs for CNS diseases fit these criteria. This is due to the fact that the water solubility of a drug is an important physical property that affects the absorption, distribution, metabolism, and excretion of drugs, as well as whether the compounds can be screened in a high-throughput manner. While it is assumed that lipophilic small molecules can be transported across the BBB, more than 98% of small-molecule drugs do not cross the BBB. Water-soluble drugs can be lipidized by blocking hydrogen-bond forming functional groups. An example is acetylation of the two hydroxyl groups in morphine to heroin, which increases BBB penetration 100-fold. However, very few CNS drugs have been developed by lipidization of water-soluble drugs, as functional groups are often metabolized in vivo. Alternatively, a water-soluble drug may be chemically modified to increase its affinity for carrier-mediated BBB transporters.
Not only must a CNS drug cross the BBB, but it must achieve therapeutic concentrations in the brain and be cleared from the brain so that the drug does not cause CNS side effects due to accumulation. As a result, CNS drugs have the highest attrition rate in development. Thus, new inhibitors of gelatinases that cross the BBB, that achieve therapeutic concentrations in the brain, and that can cleared from the brain are urgently needed to improve current therapies.